Synthesis and characterization of quarteranthene: Elucidating the characteristics of the edge state of graphene nanoribbons at the molecular level

Article abstract of DOI:10.1021/ja309599m

The characteristics of the edge state, which is a peculiar magnetic state in zigzag-edged graphene nanoribbons (ZGNRs) that originates from electron-electron correlation in an edge-localized π-state, are investigated by preparing and characterizing quarte

Full text of DOI:10.1021/ja309599m

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Article
Synthesis and Characterization of Quarteranthene:
Elucidating the Characteristics of the Edge State
of Graphene Nanoribbons at the Molecular Level
Akihito Konishi, Yasukazu Hirao, Kouzou Matsumoto, Hiroyuki Kurata, Ryohei Kishi,
Yasuteru Shigeta, Masayoshi Nakano, Kazuya Tokunaga, Kenji Kamada, and Takashi Kubo
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 Jan 2013
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Synthesis and Characterization of Quarteranthene: Elucidating
the Characteristics of the Edge State of Graphene Nanoribbons
at the Molecular Level
Akihito Konishi,† Yasukazu Hirao,† Kouzou Matsumoto,† Hiroyuki Kurata,† Ryohei, Kishi,‡
Yasuteru Shigeta,‡ Masayoshi Nakano,‡ Kazuya Tokunaga, ||,§ Kenji Kamada, ||,§
and Takashi Kubo†, *
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†Department of Chemistry, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan
‡Department of Materials Engineering Science, Graduate School of Engineering Science, Osaka University,
Toyonaka, Osaka 560-8531, Japan
||Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technol-
ogy (AIST), AIST Kansai Center, Ikeda, Osaka 563-8577, Japan
§Department of Chemistry, Graduate School of Science and Technology, Kwansei Gakuin University, Sanda, Hyogo
669-1337, Japan
ABSTRACT: The characteristics of the edge state, which is a peculiar magnetic state in zigzag-edged graphene nanorib-
bons (ZGNRs) that originates from electron–electron correlation in an edge-localized -state, is investigated by preparing
and characterizing quarteranthene molecules. The molecular geometry that was determined from the X-ray analysis is
consistent with a zigzag-edge-localized structure of unpaired electrons. The localized electrons are responsible for the
peculiar magnetic (room-temperature ferromagnetic correlation), optical (the lowest-lying doubly excited state) and
chemical (peroxide bond formation) behaviors. On the basis of these distinguishing properties and a careful consideration
of the valence bonding, insight into the edge state of ZGNRs can be gained.
(STM/S) approaches have achieved good evidence of the
INTRODUCTION
edge state around the zigzag edges in GNRs.7, 12, 13
Since its first characterization in 2004 by Geim and No-
voselov,1 research on graphene has boomed in the fields of
physics, chemistry and electronic device applications.2
Currently, a central concern in this field is how graphene
behaves when it is cut into nanoscale ribbons or flakes
with distinct open edges.3 An essential structure relevant
to this concern is graphene nanoribbons (GNRs), which
have peculiar electronic properties that are strongly influ-
enced by the edge shape.46 It has been suggested that
zigzag-edged GNRs (ZGNRs, Figure 1a) possess a localized
nonbonding -state around the zigzag edges, while arm-
chair-edged GNRs (Figure 1b) do not have such a state.
The localized nonbonding -state, which can be approxi-
mately described as a spin-polarized state according to
the broken-symmetry single-determinant approach to
multielectron correlation among unpaired electrons on
the zigzag edges, is predicted to yield specific magnetic
activities in GNRs. This peculiar electron localization,
along with the symmetry-broken spatial spin distribution
in ZGNRs, is referred to as the “edge state”,5, 6 and at-
tempts have been made to observe the edge state using
various experimental approaches.713 In particular, the
scanning tunneling microscopy and spectroscopy
These broad theoretical and experimental studies of the
edge state have clearly demonstrated that the precise fab-
rication of edge structures in GNRs is one key to under-
standing the varied behaviors of the edge state.5, 6, 12, 14, 15
Although GNRs can be prepared using chemical3a, 16, 17 and
lithographic methods18, 19 as well as through the unzipping
of carbon nanotubes,20, 21 the synthesis of perfectly termi-
nated GNRs with reliably controlled sizes has been in
high demand,22 and as such, the elucidation of the edge
state at the molecular level has become increasingly im-
portant. Polycyclic aromatic hydrocarbons (PAHs) are
possible candidates for this purpose, since they are struc-
tural components of GNRs with distinct edge structures
and have been estimated to possess a spin state similar to
those of GNRs.2326 With this in mind, we have focused on
anthenes (Figure 1c), which are rectangular PAHs with
both zigzag and armchair edges, as essential structural
elements that induce the edge state in GNRs. Our recent
experimental and theoretical studies of teranthene (2)
have afforded a glimpse of the edge state at the molecular
level and have revealed that the spin-polarized state (sin-
glet biradical state) is associated with the formation of
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(Z)-1,1',5,5'-
aromatic sextets.27, 28 This spin-polarized state is also in-
vestigated in some -conjugated small molecules having a
quinoid skeleton,29 and therefore characterizing anthene
molecules in terms of singlet biradical character would be
a reasonable approach to elucidate the characteristics of
the edge state of ZGNRs.
lowed
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the
coupling
with
tetrachlorobianthrone30 and a subsequent treatment with
SnCl2 in acetic acid gave 5. Cyclization of 5 was carried
out with KOH/quinoline to give a partially ring-closed
quinone 6.31, 32 Treatment of 6 with mesityl magnesium
bromide in the presence of CeCl333 and subsequent reduc-
tive aromatization afforded the partially ring-closed hy-
drocarbon 7a and 7b. Cyclization of 7a and 7b with
DDQ/Sc(OTf)3 followed by N2H4·H2O quench34 gave 3a
and 3b as bluish black solids, respectively. Fortunately, we
could obtain single crystals of 3a suitable for X-ray crys-
tallographic analysis by a careful recrystallization from an
o-dichlorobenzene/mesitylene solution in a degassed
sealed tube. The electronic absorption spectrum of 3a in
1,2,4-trichlorobenzene had a low-energy band centered at
917 nm with a long tail up to 1300 nm (Figure S1 in SI).
From the decay of the absorption bands, the half-life of 3a
at room temperature was determined to be only 15 hours
when exposed to air under room light.
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Here, we report the explicit observation of the edge
state using a larger anthene, quarteranthene (3), through
organic synthesis and physical measurements. We first
describe the stepwise synthesis of quarteranthene and
then the electron localization at the zigzag edges is
demonstrated by the structural analysis, physical meas-
urements and chemical reactivity of 3.
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Scheme 1. Stepwise synthetic route to quarteranthenes 3a
(Ar = mesityl) and 3b (Ar = 4-tert-butyl-m-xylyl) a
Figure 1. Chemical structure of GNRs and anthenes. (a) Zig-
zag-edged GNR (15.4 Å ribbon width). The bold line repre-
sents a quarteranthene (3) skeleton. (b) Armchair-edged
GNR (14.5 Å ribbon width). (c) Resonance structure of an-
thenes. The amount of singlet biradical character (y) was
calculated at the CASSCF(2,2)/6-31G level. The six-membered
rings depicted by bold lines represent the Clar sextets.
a
Reagents and conditions: (a) nBuLi, then tetrachlorobian-
throne. (b) SnCl2, AcOH, 58% (2 steps). (c) KOH, quinolone,
36%. (d) ArMgBr, CeCl3, THF. (e) SnCl2, AcOH, 62% (7a, 2
steps), 64% (7b, 2steps). (f) DDQ, Sc(OTf)3, 43% (3a), 57%
(3b).
Molecular geometry. The X-ray crystallographic anal-
ysis of 3a indicated two independent molecules (hereafter,
molecule A and B) in the asymmetric unit, one of which is
shown in Figure 2. Although there are small differences in
the dihedral angles of the mesityl groups (ca. 65° for mol-
RESULTS
Synthesis of quarteranthene. Scheme 1 displays the
stepwise synthetic route to 3a and 3b. Lithiation of 4 fol-
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ecule A and ca. 78° for molecule B), no appreciable differ-
ences are observed in the quarteranthene rings. The mean
width between the zigzag edges is 15.7 Å. In the packing
diagram (Figure S2 in SI), molecules A and B alternately
stack in a crossed manner with a mean interplanar dis-
tance of ~3.46 Å to form a one-dimensional chain.
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The geometry of 3a determined from the X-ray analysis
strongly suggests the localization of electrons at the zig-
zag edges. As shown in Figure 1c, the biradical resonance
contribution shortens the a bonds by increasing their
double-bond character. The mean length of the a bond in
3a is 1.414(3) Å (Figure 3), which is considerably shorter
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than those of the corresponding bonds in teranthene27
2
Figure 3. (left) Mean bond lengths (Å) in the main core ring
of 3a, (right) HOMA values of 3a.
(1.424(2) Å) and bisanthene35 1 (1.447(4) Å). This bond
shortening trend is in line with the increase in singlet
biradical character (y) estimated by the CASSCF(2,2)/6-
31G calculation, which gave a significantly high LUMO
occupation number of 0.91 for 3 in contrast to 0.54 for 2
and 0.07 for 1.36, 37 Furthermore, the harmonic oscillator
model of aromaticity (HOMA) values,38, 39 which are ge-
ometry-based aromaticity indexes, indicated more ben-
zenoid character for the peripheral six-membered rings
(the bold benzene rings in the biradical structure in Fig-
ure 1c), as shown in Figure 3. These geometric findings
suggest that the formation of aromatic sextets over-
whelms the penalty for breaking one -bond and pushes
the resulting unpaired electrons out to the zigzag edges.
Magnetic properties. The magnetic coupling of the
edge-localized electrons in 3a was estimated by perform-
ing SQUID measurements on a powdered sample (Figure
4). The measurement showed increasing susceptibility
above 50 K. From careful fitting of the observed increase,
the singlet–triplet energy gap (ES-T), that is, the magnetic
coupling strength, was determined to be 347 K (30 meV),
which is comparable to the theoretical coupling strength
of 25 meV calculated using a first-principles method for a
15-Å-wide GNR.40 This very small ES-T indicates that 3a is
easily activated to a triplet state (a ferromagnetically cor-
related state) and that the population of the triplet spe-
cies is ~50 % at room temperature.
Figure 4. χT–T plot for the powdered 3a. The measured data
are plotted as open circles. The line is the theoretical curve
calculated using the alternating Heisenberg chain model
with parameters of 2Jintra/kB = –347 K, 2Jinter/kB = 2Jintra/kB = –
151 K ( = 0.436), impurity spin contamination = 5.0%, g =
2.002 and diamagnetic susceptibility = –894.5 × 10–6 emu/mol.
The fluid toluene solution of 3b (we used 3b instead of
3a for ESR measurements because 3a becomes insoluble
at low temperatures) at room temperature gave a feature-
less broad signal centered at g = 2.002 (Figure S4 in SI).
The signal broadening would come from a large electron–
electron dipole interaction within the molecule.41 The
Figure 2. ORTEP drawings of 3a at the 50% probability level.
(left) Top view, (right) side view.
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signal intensity decreased upon cooling to 230 K and its In order to assign the absorption bands of the singlet spe-
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temperature dependency is consistent with the SQUID
measurements. We could not discern ESR signals of the
triplet species of 3b in glassy toluene at 140 K, since a Ms
= 1 peak (peak-to-peak width, Hpp = ~1 mT) would ob-
scure the triplet species signal which should have a small
D value (~0.7 mT based on the distance between the zig-
zag edges). The forbidden Ms = 2 half-field signal could
not also be observed. A triplet state signal at half-field is
known to not be observable when there is a large separa-
tion between two electrons.42
cies, we performed a strongly contracted second-order n-
electron valence state perturbation theory (NEVPT2) cal-
culation that allows multielectron excitation.44 The
NEVPT2(8,8)/6-31G* calculation revealed that the first
excited singlet state (S1) of quarteranthene is a 2Ag state
dominated by a HOMO, HOMO → LUMO, LUMO dou-
ble excitation. The excitation energy from the ground
state (S0) to S1 was estimated to be 1.32 eV (940 nm). The
S0 → S1 transition is one-photon-forbidden because of the
parity (g–g transition), and, consequently, the weak band
of 3b centered at around 1150 nm could be assigned to the
transition to the forbidden state (the breakdown of the
parity forbiddeness would come from a vibronic coupling
between the 2Ag and 1B3u (S2) states). This assignment was
confirmed by measuring the two-photon absorption
(TPA), since a two-photon excitation into an Ag state is an
allowed process. We could identify a broad band at
around 2300 nm, which is the same excitation energy as
that of the one-photon band around 1150 nm, in TPA
measurements of a CS2 solution of 3b (Figure 6).45 Thus,
the weak band around 1150 nm is associated with the sim-
ultaneous excitation of the edge-localized electrons.
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Optical properties. According to the SQUID meas-
urements, the electronic absorption spectrum of 3a
measured at room temperature should be composed of
absorption bands derived from triplet species as well as
ground-state singlet species. In order to identify the
bands of the singlet species, we performed variable-
temperature measurements. The spectrum of 3b at 303 K
exhibited a low-energy band at 920 nm along with a
shoulder band, while a new bathochromically shifted
band along with isosbestic points was observed at 970 nm
upon cooling to 183 K (Figure 5). Obviously, the batho-
chromically shifted spectrum is derived from the ground
state of quarteranthene. Interestingly, the spectra of the
singlet and triplet species are quite similar, which is unu-
sual, since most PAHs have totally different spectra in the
singlet and triplet states.43 The similarity of the spectra
may indicate similar distributions of unpaired electrons
around the zigzag edges in the singlet and triplet states.
Indeed, the UB3LYP/6-31G* calculation gives almost iden-
tical distribution patterns (except for the spin orienta-
tion) of the unpaired electrons in the singlet and triplet
state (Figure S7 in SI), where the unpaired electrons re-
side mostly on the zigzag edges.
The NEVPT2 calculation also indicated that the second
excited singlet state (S2) of quarteranthene is a 1B3u state
that possesses an almost pure HOMO → LUMO configu-
ration, and the S0 → S2 excitation energy and oscillator
strength were estimated to be 1.39 eV (888 nm) and 0.33,
respectively. The transition moment was parallel to the
long molecular axis. Therefore, the band of moderate in-
tensity observed at 980 nm for 3b is assignable to the S0
→ S2 transition. Since the highest occupied natural orbital
(HONO) and the lowest unoccupied natural orbital
(LUNO) are distributed mainly on the zigzag edges (Fig-
ure 7), it can be concluded that the band is derived from
electron transfer between the zigzag edges to afford a
charge-resonance state.
Figure 5. Electronic absorption spectra of 3b in CH2Cl2 (3.6 ×
10–5 M) for different temperatures.
Figure 6. Two-photon absorption spectrum of a CS2 solution
of 3b (1.1 × 10–3 M) under degassed condition at room tem-
perature (296 K) measured by the femtosecond Z-scan meth-
od. (2) is the two-photon absorption coefficient.
The transition to the low-lying excited states should be
related to the excitation of the edge-localized electrons.
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Chemical reactivity at the zigzag edges. The reactivi-
1
ty of quarteranthene toward oxygen is associated with the
edge-localized electrons. A toluene solution of 3b was
allowed to stand exposed to air under dark conditions at
3°C for several days (Figure 8a), which afforded an oxy-
gen-adduct one-dimensional polymer (8) in a crystalline
form. The structure of 8 was determined by X-ray crystal-
lographic analysis (Figure 8b). The oxygen molecule at-
tacks a less-protected carbon atom with a large spin den-
sity around the zigzag edge, presumably via a radical
mechanism, and forms a peroxide bond connecting the
neighbouring molecules. The HOMA analysis of 8 (Figure
9) clarified that the zigzag edges disappear after the oxy-
gen attack, and, instead, a more stable PAH skeleton that
is completely surrounded by armchair edges is generated.
The zigzag edges in nanographene are well known to be
thermodynamically unstable46, 47 and can be easily trans-
formed into the more stable armchair edges by attacks by
oxygen and various small molecules. The degradation
process of the zigzag edges in 3b may be a key element for
clarifying the degradation mechanisms of ZGNRs.
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HONO (nHONO = 1.186)
LUNO (nLUNO = 0.814)
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Figure 7. Isosurface maps and occupation numbers of
HONO and LUNO obtained from the ground state electron
density calculated at the CASSCF(8,8)/6-31G* level of approx-
imation. White/blue meshes represent the isosurfaces with
the contour values of +0.03/-0.03 au.
Figure 9. (left) HOMA values and (right) the corresponding
Clar structure of 8.
DISCUSSION AND CONCLUSION
Based on the valence bond model shown in Fig. 1c, an-
thenes are expected to behave as singlet biradical (spin-
polarized) species, since the biradical canonical structures
are reasonably stabilized by the formation of aromatic
sextets when the Kekulé structures are transformed to the
biradical structures. The contribution weight of the birad-
ical structure in the ground state increases with the mo-
lecular size as a result of the cumulative increase in the
number of aromatic sextets. Previous studies on bisan-
thene (1) and teranthene (2) revealed that bisanthene can
be categorized as a nonmagnetic species, while teran-
thene lies at the onset of the edge state. In this article, we
demonstrated that quarteranthene (3) possesses unpaired
electrons localized at the zigzag edges by examining the
molecular structure, magnetic and optical properties, and
chemical behavior. Because the physical and chemical
properties of quarteranthene are well explained by the
edge localization of the unpaired electrons and because
Figure 8. (a) Reaction of 3b with oxygen. The most domi-
nant Clar structure is drawn for 11 on the basis of the HOMA
analysis. (b) X-ray structure of 8. Ar, tBu groups and H atoms
are omitted for clarity.
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room-temperature ferromagnetic correlation was ob- san). Cyclic voltammetric measurement was conducted
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served in the SQUID measurements, quarteranthene can
be considered the minimal structural element of ZGNRs
that still shows peculiar magnetic properties. The multi-
spin correlation in ZGNRs, which is antiferromagnetic
across the ribbon and ferromagnetic along the ribbon,
could be simulated by aligning anthenes side by side.
with a BAS CV-50W electrochemical analyzer. Cyclic
voltammogram of 3a (5  10–4 M) was recorded with a
glassy carbon working electrode and a Pt counter elec-
trode in o-dichlorobenzene containing 0.1 M Bu4NClO4 as
a supporting electrolyte. The experiment employed an
Ag/AgNO3 reference electrode, and was done under a
nitrogen atmosphere at room temperature. The tempera-
ture-dependent magnetic susceptibility was measured for
randomly oriented polycrystalline samples of 3a on a
Quantum Design SQUID magnetometer MPMS-XL in the
temperature range of 2–300 K. ESR spectra were meas-
ured in a toluene solution (1 × 10–4 M) with a JEOL JES-
FE1X X-band spectrometer.
The detection of the magnetic moment around the
edges in ZGNRs has been a long-standing goal since it was
first predicted by Fujita based on the Hubbard model cal-
culation.5 Although the spin polarization itself, which
appears in broken-symmetry single-determinant calcula-
tions, is not experimentally observable in singlet systems,
the concept of spin polarization is useful for analysing the
spatial spin correlation and thus can be used for predict-
ing a real spin state realized under external perturbations.
We found that the unpaired electrons in quarteranthene
respond sharply to external perturbations such as heating,
photoirradiation and chemical reaction, which means
that quarteranthene has a prominent spin correlation in
the ground state approximately described by the spin-
polarized state and that the edge state indeed exists as an
experimentally observable state.
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10,10'-((Z)-1,1',5,5'-tetrachlorobianthrone-10,10'-
diylidene)bis(2,7-di-tert-butylanthracen-9(10H)-one)
(5): A solution of n-butyl lithium in hexane (1.6 M, 4.9
mL) was added to a solution of 427 (2.92 g, 7.3 mmol) in
ether (20 mL) at 0 ˚C under argon atmosphere. (Z)-
1,1',5,5'-tetrachlorobianthrone (1.88 g, 3.6 mmol) and THF
(15 mL) were added to the mixture after stirred at the
temperature for 15 min. The mixture was stirred for 2 days
at 0 ˚C then room temperature and quenched with glacial
acetic acid (2 mL). After solvents were evaporated from
the mixture, SnCl2 (3.10 g, 16 mmol) and glacial acetic acid
(20 mL) was added to the residue. The mixture was re-
fluxed for 3 days. After cooled, the crude product was fil-
tered and recrystallized from hexane/acetone to give 8 as
a yellow solid (2.31 g, 58%). yellow solids, m.p. (from
EtOH/CH2Cl2): >300˚C; TLC (CH2Cl2:hexane, 1:1 v/v): RF =
By using quarteranthene we are able to observe peculiar
magnetic and optical properties relevant to the correla-
tion of two unpaired electrons localized at the zigzag edg-
es. For this reason quarteranthene is an ideal system for
essentially understanding various behaviors of ZGNRs
arising from the multispin correlation. Our findings may
not only unveil the still unknown properties of ZGNRs
but also pave the way to a bottom-up approach for pro-
ducing spintronic, electronic, optical and magnetic devic-
es based on molecular-sized nanographene materials.
1
0.59; H-NMR (400 MHz, CDCl3): δ 8.26 p.p.m. (d, J = 2.4
Hz, 2H), 8.25 (d, J = 2.0 Hz, 2H), 7.82 (d, J = 8.4 Hz, 2H),
7.45 (d, J = 8.4 Hz, 2H), 7.40 (dd, J = 7.6, 1.2 Hz, 2H), 7.35
(dd, J = 8.8, 2.4 Hz, 2H), 7.31 (dd, J = 8.0, 2.0 Hz, 2H), 7.27
(dd, J = 8.0, 1.2 Hz, 2H), 7.21 (dd, J = 8.0, 1.2 Hz, 2H), 7.09
(t, J = 7.6 Hz, 2H), 6.98–7.05 (m, 4H), 1.397 (s, 18H), 1.395
(s, 18H); IR (KBr): 3064 (w), 2963 (s), 2904 (m), 2868 (m),
1667 (s), 1602 (s), 1549 (m), 1484 (m), 1444 (s), 1364 (m),
1302 (m), 1248 (s), 1143 (s), 1120 (s), 969 (s) cm-1; MS
(MALDI) (m/z): [M]+ 1097.2 [M+H]+; analysis (% calcd, %
found for C72H60Cl4O2): C (78.68, 78.52), H (5.50, 5.54).
EXPERIMENTAL SECTION
General: All experiments with moisture- or air-
sensitive compounds were performed in anhydrous sol-
vents under an argon atmosphere in well-dried glassware.
Dried solvents were prepared by distillation under argon.
Anhydrous ethanol was dried and distilled over Mg/I2.
THF was dried and distilled over sodium/benzophenone.
Dichloromethane, toluene and benzene were dried and
distilled over calcium hydride. Column chromatography
was performed with silica gel [Silica gel 60 (MERCK)].
Infrared spectra were recorded on a JASCO FT/IR-660M
spectrometer. Electronic absorption spectrum was meas-
3,7,16,20-tetra-tert-butyldianthra[9,1-fgv;9,1-
qr]bisanthene-5,18-dione (6): A solution of 5 (0.417 g,
0.380 mmol) and KOH (1.61g, 29 mmol) in quinoline (15
mL) was boiled at 190 ˚C under argon atmosphere for 2
hours. After cooled, quinoline was distilled away under
vacuum. The product was extracted with chloroform. The
organic layer was washed with hydrochloric acid, water
and brine, and dried over Na2SO4. The filtrate was evapo-
rated and the residue obtained was adsorbed by a small
amount of silica gel. Then, the mass was put on to on
alumina column chromatography and eluted with CH2Cl2
and CHCl3:CH2Cl2 (1:1 v/v) to give 6 as a reddish purple
solid (0.129 g, 36%). Reddish purple solids, m.p. (from
1
ured with a JASCO V-570 spectrometer. H NMR spectra
were obtained on JEOL EX-270, Bruker Mercury300 and
JEOL GSX400 spectrometers. Positive EI, MALDI-TOF
and ESI mass spectra were taken by using Shimadzu QP-
5050, Shimadzu AXIMA-CFR, Applied Biosystems Japan
Ltd. QSTAR Elite and ThermoFisher Scientific LTQ
ORBITRAP XL mass spectrometers, respectively. High
resolution mass spectrum was analyzed by using Applied
Biosystems Japan Ltd. Analyst QS 2.0 and ThermoXcali-
bur 2.1.0.1140. Data collection for X-ray crystal analysis
was performed on Rigaku/Varimax diffractometer (Mo-
K,  = 0.71075 Å). The structure was solved with direct
methods and refined with full-matrix least squares (teX-
1
CH2Cl2): > 300˚C; TLC(CH2Cl2): RF = 0.56; H-NMR (300
MHz, CDCl3): δ 9.18 p.p.m. (d, J = 1.5 Hz, 2H), 9.09 (d, J =
1.2 Hz, 2H), 8.85 (d, J = 9.6 Hz, 2H), 8.63(d, J = 9.0 Hz, 2H),
8.41–8.45 (m, 6H), 7.80 (d, J = 8.7 Hz. 2H), 7.31 (d, J = 6.6
Hz, 2H), 7.11 (dd, J = 8.7, 2.7 Hz, 2H), 1.75 (s, 18H), 1.36 (s,
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Journal of the American Chemical Society
18H); IR (KBr): 3069 (w), 2957 (s), 2903 (m), 2867 (m),
ammetry) E2ox = –0.17, E1ox = –0.41, E1red = –1.47, E2red = –1.59
V vs. Fc/Fc+, see also Figure S3 in SI.
1
2
3
4
5
6
7
8
1654 (s), 1597 (s), 1567 (m), 1482 (m), 1362 (m), 1251 (s),1050
(w), 886 (w), 840 (m), 802 (s), 757 (s), 663 (m) cm-1; MS
(MALDI) (m/z): 952.5 [M] +, 953.5 [M+H]+; HRMS (ESI)
(m/z): [M+H]+ calcd for C72H57O2, 953.4353; found,
953.4331.
2,6,13,17-tetra-tert-butyl-4,15-di(4-tert-butyl-2,6-
dimethylphenyl)quarteranthene (3b): The precursor
7b (0.062 g, 0.050 mmol), DDQ (0.066 g, 0.29 mmol) and
Sc(OTf)3 (0.189 g, 0.38 mmol) were dissolved in anhy-
drous toluene (20 mL), and the mixture was refluxed for
84 h under argon atmosphere. After the mixture was
cooled to 0˚C, Et3N (0.2 mL) and N2H4•H2O (0.1 mL) were
added to the mixture, and the mixture was stirred for 10
min at the temperature. The reaction mixture was poured
onto hydrous (6%) alumina and the product was eluted
with toluene at –20˚C under argon flows to give 3b as a
bluish black solid (0.035 g, 57%). m.p. (from toluene):
>300˚C; IR (KBr): 3048(w), 2954 (s), 2905 (s), 2866 (m),
1739 (w), 1606 (w), 1565 (m), 1479 (m), 1460 (w), 1393 (w),
1361 (m), 1226 (w), 968 (w), 868 (w), 793 (m), 754 (w), 659
(w) cm-1; HRMS (ESI) (m/z): [M]+ calcd for C96H86:
1238.6724; found, 1238.6715.
3,7,16,20-tetra-tert-butyl-5,18-diaryldianthra[9,1-
fg;9,1-qr]bisanthene (7a and 7b): A solution of aryl
magnesium bromide (6.7 mmol) in THF (10 mL) was add-
ed to the suspension of cerium trichloride (2.35 g, 7.2
mmol) in THF (40 mL) at 0˚C under argon atmosphere
and the mixture was stirred for 1.5 h at the temperature.
After the mixture was cooled to –40 – –50˚C, a solution of
6 (0.120 g, 0.126 mmol) in THF (10mL) was added to the
mixture. The mixture was stirred for 2 h at the tempera-
ture and quenched with aq. NH4Cl.The product was ex-
tracted with CH2Cl2 and the organic layer was washed
with water and brine, and dried over Na2SO4. The filtrate
was evaporated and the residue obtained was dissolved in
anhydrous toluene (2 mL). SnCl2 (0.30 g, 1.6 mmol) was
added to the toluene solution and stirred for 15 min at
room temperature under argon atmosphere. After stirring,
hexane was added to the reaction mixture and the solu-
tion was poured onto hydrous (6%) alumina. The product
was eluted with toluene: hexane (1:1 v/v) to give 7 as a
reddish purple solid (a: R= methyl 0.092 g (62%), b: R =
tert-butyl 0.119 g (64%))
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Two-photon absorption measurements: Two-
photon absorption spectrum was measured by using the
open-aperture Z-scan method.48 The experimental setup
was reported previously.49 A femtosecond optical para-
metric amplifier (Spectra-Physics OPA-800) operating at
a repetition rate of 1 kHz was used as light source to scan
the excitation wavelength form 1680 – 2420 nm. The out-
put beam of the parametric amplifier passed through a
small iris to obtain near Gaussian spatial profile. The
pulsewidth was measured at each wavelength by an auto-
correlator (typically 110 fs in FWHM for Gaussian pulse)
and used to calculate the peak optical intensity at the
focal point I0. The Rayleigh range zR of the optical setup
was 5–11 mm depending on the wavelengths employed.
The sample solution (3b in CS2, 1.1 mM) was held in a
quartz cuvette (path length L = 1 mm), which is shorter
than the zR and satisfied the “optically thin” condition L
<< zR.48 The transmitted light through the sample was
detected by an InGaAs-photodiode attached to an inte-
grating sphere. Measurements were repeated at least four
times by varying the incident laser power Pi in the range
of 0.1 – 1 mW at each wavelength.
3,7,16,20-tetra-tert-butyl-5,18-dimesityldianthra[9,1-
fg;
9,1-qr]bisanthene
(7a):
m.p.:
>
300˚C;
TLC(toluene:hexane, 1:1 v/v): RF = 0.59; IR (KBr): 3066 (w),
2960 (s), 2864 (s), 1611 (w), 1587 (w), 1477 (m), 1460 (s),
1362 (s), 1256 (m), 942 (s), 849 (w) cm-1; MS (MALDI)
(m/z): 1159.3 [M+H]+ ; HRMS (ESI) (m/z): [M]+ calcd for
C90H78, 1158.6098; found, 1158.6109.
3,7,16,20-tetra-tert-butyl-5,18-di(4-tert-butyl-2,6-
dimethylphenyl)dianthra[9,1-fg;9,1-qr]bisanthene
(7b): m.p.: >300˚C; IR (KBr): 3066 (w), 2959 (s), 2910 (s),
2865 (s), 1588 (w), 1478 (m), 1459 (m), 1361 (m), 1258 (m),
958 (m), 803 (m) cm-1; HRMS (ESI) (m/z): [M]+ calcd for
C96H90: 1242.7037, found:1242.7045.
2,6,13,17-tetra-tert-butyl-4,15-
The recorded Z-scan traces were analyzed by the curve
fitting procedure with the theoretical model of the trans-
mittance of a spatially and temporally Gaussian pulse
through two-photon absorptive media.49 From the curve
dimesitylquarteranthene (3a): The precursor 7a (0.091
g, 0.078 mmol), DDQ (0.093 g, 0.41 mmol) and Sc(OTf)3
(0.276 g, 0.60 mmol) were dissolved in anhydrous toluene
(20 mL), and the mixture was refluxed for 84 h under ar-
gon atmosphere. After the mixture was cooled to room
temperature, Et3N (0.2 mL) and N2H4·H2O (0.1 mL) were
added to the mixture, and the mixture was stirred for 10
min at the temperature. The reaction mixture was poured
onto hydrous (6%) alumina and the product was eluted
with toluene to give 3a as a bluish black solid (0.039 g,
43%). Single crystals of 3a were obtained by the recrystal-
lization from a mesitylene/o-dichlorobenzene solution.
m.p. (from mesitylene/o-dichlorobenzene): >300˚C; IR
(KBr): 3075(w), 2954 (s), 2905 (s), 2866 (m), 1610 (w), 1566
(m), 1476 (m), 1461 (w), 1437 (w), 1361 (w), 1261 (w), 1020
(w), 969 (w), 835 (w), 794 (s), 740 (w), 659 (w) cm-1; MS
(MALDI) (m/z): 1155.0 [M]+; redox potentials (cyclic volt-
fitting the two-photon absorbance of the sample q0
=
(2)I0L was obtained, where (2) is the two-photon absorp-
tion coefficient. The proportionality relation of q0 against
I0, therefore Pi, was confirmed, which verified that the
observed signal surely arose from the two-photon absorp-
tion process. The I0 employed was less than 200 GW cm-2.
Two-photon absorption spectrum (Figure 6) of the sam-
ple was obtained by repeating the measurement at each
wavelength, where the linear absorption of both solute
and solvent is absent.
Computational detail: All ab initio calculations were
performed with the Gaussian 03 program50 and the
Molpro Version 2010.1 program package.51 The molecular
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Journal of the American Chemical Society
Page 8 of 10
(7) Kobayashi, Y.; Fukui, K.; Enoki, T.; Kusakabe, K; Kaburagi, Y.
geometries of 1, 2 and 3 used for the CASSCF and NEVPT2
calculations were optimized at the UB3LYP/6-31G* level
of theory. The aryl and tert-butyl substituent groups were
replaced by hydrogen atoms.
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1
2
3
4
5
6
7
8
Crystal data for compound 3a: T = 200(2) K, mono-
clinic, space group P21/n (No. 14), a = 9.3333(7), b =
36.817(2), c = 19.2308(14) Å, = 96.9458(17)˚, V =
6559.6(8) Å3, Z = 4, R1 (wR2) = 0.0853 (0.2470) for 931 pa-
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Crystal data for compound 8: T = 200(2) K, triclinic,
space group P-1 (No. 2), a = 9.7396(3), b = 19.3986(7), c =
21.6326(8) Å,  = 77.6474(11),  = 89.1137(9),  =
84.1088(11)˚, V = 3971.4(2) Å3, Z = 2, R1 (wR2) = 0.0761
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ASSOCIATED CONTENT
Supporting Information. Electronic absorption spectrum,
cyclic voltammogram, and packing diagram of 3a. ESR and
1
variable temperature H-NMR spectra of 3b. Computational
odd electron density and spin density maps of 3. Details on
the CASSCF and NEVPT2 calculations. CIF file of 3a and 8.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(21) Jiao, L.; Wang, X.; Diankov, G.; Wang, H.; Dai, H. Nat. Nano-
technol. 2010, 5, 321–325.
AUTHOR INFORMATION
(22) Very recently Hong et al. have prepared zigzag edges by the
direct chemical functionalization of graphene, which allows the
direct observation of room temperature magnetic ordering in 30
Å wide ZGNRs. See, Hong, J.; Bekyarova, E.; Liang, P.; de
Heer, W. A.; Haddon, R. C.; Khizroev, S., Sci. Rep. 2012, 2,
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Champagne, B.; Shiomi, D.; Sato, K.; Takui, T.; Matsumoto,
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Corresponding Author
kubo@chem.sci.osaka-u.ac.jp
ACKNOWLEDGMENT
This work was supported in part by the Grant-in-Aid for Sci-
entific Research on Innovative Areas “Reaction Integration”
(No. 2105) from the Ministry of Education, Culture, Sports,
and Technology of Japan and by the ALCA Program of the
Japan Science and Technology Agency (JST). A. K. acknowl-
edges support from the JSPS Fellowship for Young Scientists.
The authors thank Y. Aso and Y. Ie (the Institute of Science
and Industrial Research, Osaka University) for the variable-
temperature electronic absorption measurements and K.
Wakabayashi (National Institute for Materials Science,
NIMS) for fruitful discussions and helpful comments.
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Journal of the American Chemical Society
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Journal of the American Chemical Society
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